 You've got a point there . It is a custom and good manners to serve tea to a guest who comes to pay a visit . There is no need to ask if he needs it or not , or if he wants something else . It might be an insult to the guest if he is left sitting there without being served tea . 
 So the guest is presumed to be thirsty and in need of some drink after the trip . In that case , I guess , in return , the guest doesn't have to drink it if he wouldn't like it . 